1. Field of the Invention
The present invention relates to a solid-state image sensor and, more particularly, to a solid-state image sensor and a fabricating method thereof in which poly gates in a horizontal charge coupled device (hereinafter referred to as HCCD) are made to have different lengths to omit a barrier ion implanting process step, thus simplifying the entire process and maximizing the charge-transferring efficiency.
2. Discussion of the Related Art
FIG. 1 is a layout of a general solid-state image sensor which includes a plurality of photo diode (PD) regions for converting a light signal into an electric signal charge, vertical charge coupled device (hereinafter referred to as VCCD) regions formed at right angle to the PD regions for transferring in a vertical direction the electric signal charge converted by the PD regions, a HCCD formed at right angle to the VCCDs for transferring in a horizontal direction the signal charge transferred from the VCCDs, and a sensing amplifier SA for sensing the signal charge transferred from the HCCD.
In such a solid-state image sensor having the aforementioned structure, since the HCCD must read charges in parallel transferred from VCCDs in a very short time, the corresponding clocking should be carried out fast. Accordingly, a 2-phase clocking is carried out on average in an HCCD unlike VCCDs in which a 4-phase clocking is carried out.
A conventional HCCD will be described with reference to the accompanying drawings. FIG. 2A is a cross-sectional view of a conventional HCCD and FIG. 2B is a potential profile of a conventional HCCD.
As shown in FIG. 2A, a HCCD includes a p-type well 13 formed on an N-type semiconductor substrate 11, a buried charge coupled device (hereinafter referred to as BCCD) 15 formed on a predetermined area of the p-type well 13, a gate insulating layer 17 formed on the BCCD 15, first and second polygate electrodes 19 and 19a formed alternately and insulatively on the gate insulating layer 17, and buried regions 21 formed under the second polygate electrodes 19a for potential difference between the second polygate electrodes 19a and the BCCD 15. At this time, a gate insulating layer 17a is formed between the first and second polygate electrodes 19 and 19a.
In this HCCD, with potential wells maintained in a form of stairs as shown in FIG. 2B, charges are moved in a specific direction. To describe in more details, when t=1, charges gather at the bottom of the potential well of a fourth polygate electrode which a high voltage is applied to. When t=2, a high voltage is applied to the first and second polygate electrodes, whereby energy levels at the bottoms of the first and second polygate electrodes decrease; and a low voltage is applied to the third and fourth polygate electrodes, whereby energy levels at their bottoms rise. However, electrons which gather at the bottom of the fourth polygate electrode can not move left because of a barrier region 21 under the third polygate electrode. As energy levels of fifth and sixth polygate electrodes decrease gradually such that a right energy barrier of the fifth polygate electrode is removed, charges moved toward the bottom of the fifth and sixth polygate electrodes having low energy levels. If the energy levels of the fifth and sixth polygate electrodes become high enough, potential walls having a form of stairs are formed such that the electrons-gathering position turns from the bottom of the fourth polygate electrode to that of the eighth polygate electrode. As previously described, potential levels change sequentially by using the conventional 2-phase clocking, thereby moving signal charges toward the sensing amplifier.
However, such a conventional solid-state image sensor has the following problems. Since the potential difference is achieved after formation of a first polygate electrode, an ion-implanting process step is performed with the first polygate electrode serving as a mask. Further, since another process step is necessary for forming a second polygate electrode, the entire process is very complex.